1. Field of the Invention
The present invention relates to a camera having a shutter instrumentation device to detect the state of travel of a camera shutter.
2. Description of the Related Art
In conventional cameras, light beams from a photographic subject passing through a camera aperture are limited by a stop and the time for exposure of film is limited by a mechanical shutter. A focal plane shutter used in a single lens reflex camera basically consists of two blinds, a front blind and a rear blind. To control film exposure to the light beams from the photographic subject, the front and rear blinds cover the film picture plane so that the film is not exposed.
The front blind initially blocks the aperture, thereby preventing light from reaching the film picture plane. When a photographer presses a release button, the front blind is moved away from the aperture, and exposure of the film picture plane is commenced. After a predetermined time has elapsed, the rear blind moves in a position so that it blocks the aperture and covers the film picture plane. The travel of the front blind and the rear blind is performed mechanically, energized by spring force, and the commencement of travel is performed by releasing a mechanical hold.
Typically, the front and rear blind operations are controlled purely mechanically. However, electrical control has become used in recent years. With electrical control, spring force is used to drive the travel of the front blind and the rear blind, but the engagement and disengagement of a mechanical hold of the front and rear blinds are controlled by passing current to an electromagnet.
Moreover, there is a need to increase the speed of a focal plane shutter. For example, shutter speeds of 1/8,000 second are desirable. In addition, there is a widespread tendency to use strobe, thereby requiring an increase in the synchronized speed of focal plane shutters to allow photography in bright surroundings.
In order to increase the synchronous speed of focal plane shutters, the spring force controlling the front and rear blinds is increased and the front and rear blinds are moved in rapid succession. In addition, by rapidly moving the front and rear blinds, a slit of narrow width is formed. The width of the slit can be controlled by the overlapping motion of the front and rear blinds.
The present invention recognizes that, even with accurate timing of current flow in the electromagnet, the film exposure time cannot be accurately controlled. The actual exposure time depends only on the mechanical travel of the front blind and the rear blind. As displacement elements, there is scatter in the speed of separation of the hold mechanism and the mechanical travel system (which includes a spring). Moreover, these mechanical elements inevitably have varying characteristics due to temperature characteristics or changes with time. These problems become more prevalent when blind travel speeds are increased.
When in an anomalous state, the desired shutter speed is not obtained and the amount of exposure of the film surface becomes excessively insufficient. In extreme cases, the front blind and the rear blind overlap as they travel, with the result that the film is not exposed at all. A converse phenomenon can occur if the blinds do not close, thereby resulting in overexposure.
These problems are especially prevalent in a camera with a shutter having a fast blind speed and narrow slit width.
Also, if any of the above problems occur during photography, the problems cannot be detected until the film is developed. This poses a serious problem since a photographer could continue to take photographs without realizing that the film is not being properly exposed.
Conventional cameras have attempted to determine the shutter state by forming a capacitor with a shutter vane as a "dielectric" between two plates. In this manner, the capacitance is different when the shutter vane is between the two capacitor plates compared to when the shutter vane is not between the two plates. This approach, however, can be inaccurate.